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\bibitem{Massobrio_etal2013} {\textquotedblleft}\textit{Stability of Ge$_{12}$C$_{48}$ and Ge$_{20}$C$_{40}$ heterofullerenes : a first principles molecular dynamics study}{\textquotedblright}. Massobrio C, Djimbi DM, Matsubara M, Scipioni R, Boero M, Chemical physics letters \textbf{556}, 163 (2013).   http://doi.org/10.1016/j.cplett.2012.11.033

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